Engine emission control systems may utilize various exhaust sensors. One example sensor may be a particulate matter sensor which indicates particulate matter mass and/or concentration in the exhaust gas. In one example, the particulate matter sensor may operate by accumulating particulate matter over time and providing an indication of the degree of accumulation as a measure of exhaust particulate matter levels.
Accuracy of particulate matter sensors may be affected by non-uniform deposition of soot on the sensor due to a bias in flow distribution across the surface of the sensor. Further, particulate matter sensors may be prone to contamination from an impingement of water droplets and/or larger particulates present in the exhaust gases. This contamination may lead to errors in sensor output. Furthermore, sensor regeneration may be inadequate when a substantial quantity of exhaust gases stream across the particulate matter sensor.
The inventors herein have recognized the above issues and identified an approach to at least partly address the issues. In one example approach, a particulate matter assembly for sensing particulate matter in an exhaust passage of an engine is provided. The particulate matter assembly comprises an outer stepped tube having a first semi-circular region and a second semi-circular region, the first region being longer than the second region, an inner stepped tube coaxial to the outer stepped tube having a third semi-circular region and a fourth semi-circular region, the third region being longer than the fourth region, and a plate positioned inside the inner stepped tube having a sensor element. In this way, by coupling the outer stepped tube and the inner stepped tube, a stepped structure (hereafter referred to as a step) may be generated in the PM sensor assembly. In addition, inlets may be formed on the step to direct exhaust gas into the PM sensor assembly. As an example, inlets positioned along the step may direct exhaust in the exhaust passage in a direction opposite to the direction of exhaust flow in the exhaust passage.
As such, larger particulates in the exhaust may not be able to sufficiently reverse the flow direction and enter the PM sensor assembly via the step. Thus, the step serves to substantially block the larger particulates in the exhaust flow from impinging on the sensor element formed on the plate positioned within the assembly, thereby reducing fluctuations at the sensor due to large particulates depositing on the sensor element.
As one example, an exhaust particulate matter sensor assembly may be positioned downstream of a particulate filter in an exhaust pipe. The particulate matter sensor assembly may include a protection tube comprising an outer asymmetrical tube composed of semi-circular regions of unequal lengths. Likewise, the inner tube may include asymmetrical semi-circular regions that are positioned entirely within the outer tube. When coupled together, the difference in lengths of the asymmetrical semi-circular regions of each of the outer tube, and the inner tube may result in a step structure being created on one face of the sensor assembly. As such, the step may include inlets to direct the exhaust gas into an annular region formed between the inner and the outer stepped tube. As a consequence, the exhaust gas may be able to enter the PM sensor via the inlets in the slit by undergoing a reversal in the direction of flow. However, larger particulates and water droplets may not be able to sufficiently reverse the flow direction in order to be able enter the inlets on the step. Thus, larger particulates and/or water droplets may be blocked by the step, reducing sensor errors. Further, sensor elements positioned within the inner tube may experience a more uniform soot deposition through a direct flow impingement on the surface of the electrodes.
In this way, the step formed as a result of the asymmetry in the protection tube design may block the larger particulates and/or water droplets from entering the particulate matter sensor assembly. The technical effect of including an asymmetrical protection tube and a stepped structure in the design of the particulate matter sensor assembly is that the sensor element positioned within may be better protected from impingement of larger particulates and contaminants without adding additional components and/or filters to the particulate matter sensor assembly. Overall, the functioning of the sensor element may be improved and the sensor may be rendered more reliable.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.